The invention concerns a hybrid photonic-electronic subscriber access unit for connecting optical subscriber lines asynchronous transfer mode telecommunication network. Optical subscriber lines convey data in the form of fixed length cells including a header containing routing information.
French patent application no. 2 646 036 describes an entirely electronic digital subscriber access unit including: a control station, an asynchronous transfer mode switching network and a plurality of concentrators for connecting subscriber line terminals, including asynchronous time-division multiplex terminals. Each such prior art concentrator includes a plurality of subscriber line interface circuits and two concentration stages. Each subscriber line interface circuit sends and receives signalling and maintenance cells.
The first concentration stage includes an asynchronous time-division switching matrix controlled by a microprocessor. The second concentration stage includes another asynchronous time-division switching matrix or a time-division multiplexer-demultiplexer controlled by a microprocessor. The structure of this concentrator is optimized for the use of electronic components.
French patent application no. 2 672 175 describes a mainly photonic subscriber access unit. It multiplexes and routes cells using photonic components along all of their path between an input and an output of the subscriber access unit, but these components are controlled by control devices constructed from electronic components. The photonic technology is used for multiplexing, bit rate conversion and routing operations, which require only a small number of components for a given capacity as photonic components can process cells at bit rates of 2.6 Gigabit/second using wavelength-division multiplexing on 16 wavelengths, for example.
This subscriber access unit includes:
a photonic switching network connected to a parent central office; PA0 an electronic control unit connected to a switching network; PA0 a plurality of mainly photonic concentrators connected to the switching network; PA0 mainly photonic subscriber line interface circuits connected to the subscriber lines and to the concentrators, respectively. PA0 cell delineation; PA0 descrambling the data field of each cell; PA0 extracting empty cells. PA0 inserting empty cells; PA0 scrambling the data field; PA0 calculating an error detection word. PA0 an asynchronous transfer mode switching network; PA0 subscriber line interface circuits connected to respective subscriber lines; and PA0 at least one concentrator connecting the switching network to the subscriber line interface circuits; PA0 and is characterized in that the subscriber line interface circuits include: PA0 electronic means for discriminating between empty cells, signalling cells and maintenance cells in the stream of cells from the subscriber lines; PA0 electronic means for translating routing information contained in the headers of the cells from the subscriber lines and writing the translated information into said headers; PA0 electronic means for adding a routing label to the cells from the subscriber lines; and PA0 electronic means for monitoring the bit rate of the cells from the subscriber lines. PA0 means for statistically time-division multiplexing the cells from the subscriber line interface circuits; PA0 photonic means for wavelength-division multiplexing the cells output by the electronic statistical time-division multiplexing means; PA0 photonic means for multiplying the bit rate of the cells output by the wavelength-division multiplexing means; and PA0 photonic means for time-division multiplexing without wavelength-division multiplexing the cells output by the bit rate multiplying means.
Each subscriber line interface circuit performs the following functions in respect of cells incoming on an optical subscriber line:
It performs the following functions in respect of cells addressed to a subscriber line:
The means for translating a virtual circuit label or virtual circuit group label and for attaching a routing label to each cell incoming on a subscriber line are not located in the subscriber line interface circuits or in the concentrators. Instead they are at the input of the switching network and are therefore pooled to process cells incoming on all the subscriber lines connected to one or more concentrators. The means implementing the policing function which monitors the bit rate are also located at the input of the switching network and are also pooled for processing of cells from all subscriber lines connected to one or more concentrators. These policing means can therefore be shared by a large number of subscriber lines, 256 lines, for example.
The number of photonic components in a subscriber access unit constructed in this manner is therefore very significantly reduced as compared with the same design of entirely electronic subscriber access unit, by replacing electronic components with photonic components. This design is therefore highly advantageous for entirely photonic subscriber access units, i.e. subscriber access units with photonic control devices which can process data contained in cells with a bit rate of 2.6 Gb/s without the bit rate causing any problems.
At present the cost of optical logic gates is such that optical processors and high-capacity optical memories cannot be produced at acceptable cost. It is therefore necessary to use electronic processors and memories to implement the functions mentioned above. As this design of subscriber access unit entails carrying out logic operations on cells at a bit rate of 2.6 Gigabits/second it is necessary to convert the bits to be processed to parallel form, in order to change their bit rate, and to use many electronic components in parallel. This increase in the number of electronic components leads to a significant additional cost.